The invention relates to a wire electrode for spark erosion cutting of metals, electrically-conducting ceramics etc., comprising either a homogenous centre made of a metal or of a metal alloy, or a composite centre and comprising at least one sheath coating which wears during erosion.
When using wire electrodes it has been shown that wire electrodes comprising sheath coatings, wherein the sheath coating for example comprises xcex2-brass or xcex3-brass, have a better cutting performance than electrodes without such sheath coatings. This is due to the fact that the xcex2/xcex3-sheath coating comprises a high zinc content which is embedded in the microstructure. Such wire electrodes comprising xcex2-brass sheath coatings or xcex3-brass sheath coatings have a disadvantage in that the sheath coating with its cubic body-centred lattice is significantly less well deformable than the centre which usually comprises a cubic face-centred xcex1-brass. This results in the surface of such a sheath electrode ripping up in sections. During this process, cracks form which, as has been shown in investigations, have a certain advantage in some respect, because this obviously supports clearing the cutting gap. Consequently, in the case of higher workpieces, improved cutting performance is achievable with such electrodes comprising a rough surface. Such wire electrodes are however associated with a disadvantage in that due to the irregular and fissured hard surface, both guidance of the wire and current conduction to the wire, are impeded. Such a sheath coating is very rough and hard, so that guides wear relatively quickly. Furthermore, vibrations in the wire are generated which also further impede current conduction. These vibrations are also disadvantageous for guiding the wire in the gap because the wire electrode is excited to produce oscillations which can lead to short circuits.
It is the object of the invention to improve a wire electrode of the type mentioned in the introduction so that the advantages of improved cutting performance obtained by the fissured surface of the sheath coating, are maintained, without the disadvantages of diminished current transition and increased wear and increased oscillations in the wire, having to be accepted. It is a further object of the invention to provide such an electrode with a surface structure which leads to a further improvement in performance.
According to the invention, this object is met in that the surface of the outer sheath coating is structured and in that the indentations created by the structure are filled in with a filler comprising an easily vapourable metal or a metal alloy for obtaining a surface with a short peak-to-valley height and thus little surface roughness.
If the fissures in the sheath coating, created during the manufacture of coated wire electrodes, are filled in with an easily vapourable metal or a metal alloy comprising zinc or a zinc alloy, with the zinc fraction in the alloy being significantly above the zinc fraction in the sheath coating, then a wire electrode with a smooth outside surface is obtained which can be transported in the wire guides with only slight wear, wherein the entire contacting surface can be used for current conduction, with irregularities which can cause vibrations and which impede current conduction, being avoided. In this way it is possible to considerably increase the current feed to the electrode. During the subsequent erosion process in the cutting gap, the filler which is present in the fissures or grooves is used up first so that along most of the length of the cutting gap a sheath coating is available for the erosion process, which sheath coating comprises a strongly fissured surface. As a result of this fissured surface, the discharge of the eroded material is improved while at the same time field intensity peaks and thus locations of increased discharge probability and associated with this an increase in the discharge frequency, are generated by the projections.
During normal manufacture of wire, during which due to the different deformation behaviour of centre and sheath coating, fissures are produced in the sheath coating, the above-mentioned locations with increased field intensity peaks are generated purely at random. However, if the surface of the sheath coating comprises grooves which can be shaped so as to be longitudinal, transverse, helicoidal or intersecting, then such locations with an increased discharge probability are provided. This significantly accelerates the erosion process also for the reason that here there is a significant increase in the probability of discharge not occurring in the crater of a discharge which has been generated shortly beforehand. Thus, an even discharge distribution is achieved as a result of the structured surface. At the same time this results in a reduction in the risk of the wire breaking. These grooves or indentations are at least 2 xcexcm in depth and at most 50 xcexcm in depth.
A further advantage achieved, namely that of preventing local discharge accumulation, consists of the discharges being more often enclosed by a liquid dielectric instead of a gas bubble. As a result of the associated more pronounced contraction of the plasma channel, there is greater materials removal per discharge, which clearly has a positive effect on the cutting performance of the wire electrode.